The present invention relates to a process for producing a foam ceramic.
Foam ceramics contain air bubbles, so they have lower density and strength than conventional ceramics, even if the latter have pores. The pore content (“porosity”) corresponds to the ratio of hollow space volume to total volume; for example in the case of bricks—e.g., insulating firebricks—it is about 10% to 80%, depending on the raw materials and how they are fired. By contrast, as a rule foam ceramics have a porosity of more than 90% and pores that are produced in a controlled manner.
Foamed mineral materials for construction and other purposes (e.g., foam ceramic) are produced today, for example, in the form of foamed concretes for thermal insulation (see WO 2011/044605 A1 or DE 197 08 799 A1), however, they achieve only very low strength, and therefore are not widely used. Other foam ceramics with a macroporous, open-pored foam structure, such as described, for example, in WO 2013/137727 A1, are used in the filtration of metals.
Three processes are generally known for the controlled production of pores inside fired ceramics: First, it is possible, before firing a free-flowing raw material, for it to be mixed with a porosifying agent, which burns out during firing, leaving behind pores whose particle size and shape correspond to those of the porosifying agent; however, the formation of gas when the porosifying agent is burned out presents the danger of cracking. Second, the free-flowing raw material suspension can be taken up by a structured (open-pored) template, so that the latter is replicated when it is burned out (“replica technique”); however, this process can only be used to produce open-pored foam ceramics that allow flow-through, which are less suitable for insulating materials. Third, direct foaming of the free-flowing raw material suspension can produce a wet foam, and subsequent firing can produce a foam ceramic; the wet foam is quite unstable, so in this case it is difficult to produce pores in a controlled manner.